It should be [HBr][Br2] in the denominator.
So if [Br2] is very less, then the term k2 [HBr][Br2] can be neglected compared to 1. So the reaction becomes 2nd order. So C is correct. Similarly B is correct.
Clearly, the rate law is not k1 [A][B] which is in case of elementary reaction. So it is a complex reaction and hence not single step.
Since molecularity of complex reaction is not defined, option D does not make sense.